Production of metals



United StatesPatentO PRODUCTION OF METALS John L. Ham, Wellesley Hills,and David I. Sinizer, Bedford, Mass, assignors to National ResearchCorporation, Cambridge, Mass., a corporation of Massachusetts NoDrawing. Application March 30, 1956 Serial No. 574,982

2 Claims. (Cl. 75-10) This invention relates to techniques for themelting and casting of refractory metals such as titanium, zirconium andthe like and theiralloys. Although the invention is primarily directedtoward the melting and casting of refractory metals, it can equally wellbe utilized in connection with any highly reactive metal or alloy, suchas uranium-niobium alloys, for example. For the purposes of simplicity,only titanium will be specifically referred to in the remainerof theapplication, but it isnot to be construed that the invention is limitedto titanium alone.

In the past, it has been extremely difficult to produce ingots oftitanium ortitanium alloys that were both homogeneous and free ofhydrogen and occluded gases Moreover, the use of scrap titanium in theproduction of such ingots involved many expensive steps which weredifficult to perform and which tended to obviate the advantages to begained from the use of this relatively inexpensive starting material.

In the instant invention, the disadvantages of the prior method havebeen successfully resolved or circumvented.

Accordingly, it is a principal object of the present invention toproduce homogeneous ingots of titanium and titanium alloys.

Another object of the invention is to produce homogeneous ingots oftitanium and titanium alloys which are substantially free of occludedgases.

Still another object of the invention is to provide an improved methodof utilizing scrap titanium in the production of homogeneous,hydrogen-free titanium ingots.

Still another object of the invention is to provide a process in'whichscrap titanium, ranging in size from minute particles to pieces weighingseveral pounds or more, can be melted into homogeneous, hydrogen-freeingots.

Still another object of the invention is to produce alloys within closetolerances with respect to both substitutional and interstitialelements.

Still another object of the invention is to control the hydrogen contentin ingots of titanium and titanium alloys.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process involving the severalsteps and the relation and the order of one or more such steps withrespect to each of the others which are exemplified in the followingdetailed disclosure, and the scope of the application of which will beindicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description.

In the prior art, because of the high reactivity of molten titanium, themelting of titanium by standard induction techniques in conventionalcrucibles has been superseded by cold-mold arcmelting with eitherconsumable or nonconsumable electrodes.

Nonconsumable cold-mold melting is subject to the disadvantages ofchemical inhomogeneity, tungsten inclusions or carbon contamination(depending upon the electrode used), restricted size of the chargematerial, and the possibility of highhydrogen content, although thislast disadvantage can be minimized by flowing argon through the furnacein order to maintain a constant pressure of argon in the system whilemelting.

. Consumable electrode cold-mold melting has been used to advantageutilizing either first-melted ingots or briquetted sponge blended withalloying elements as the electrode. Homogeneity and uniformity of ingotscan be achieved by these means, but use of this technique has proved tobe disadvantageously expensive.

In the present invention, a highly reactive refractory metal such astitanium is mixed with an alloying metal in a skull comprising the basemetal or alloy. The mixtureis then melted to form a pool of ahomogeneous alloy, and the molten alloy is poured'into a mold to form aconsumable electrode of the alloy, which consumable electrode issubsequently arc melted under avacuum in a cold-moldfurnace to produce ahydrogen-free homogeneous alloy ingot. In skull melting,the liquid meltis contained in a solid shell or. skullof titanium or titanium alloy,since no other known material will contain the melt without chemicalreaction. 1

In a preferred embodiment of the invention, the skull is contained in acrucible which is preferably cooled by radiation to the furnace walls.The skull thickness is determined by the power input and the coolingrate.

Melting can be done by D. C. are using nonconsumable tained molten for asufiicient time to achieve homogeneity, 4

the molten metal is poured into a chill mold in order to form ingotswhich are utilized subsequently as consumable electrodes. Theseconsumable electrodes are then used in a cold-mold arc-melting processin which themetal or alloy is substantially completely degassed and ahomogeneous ingot is obtained. This ingot is also substantiallyhydrogen-free, a hydrogen content of below 50 p. p. in. being easilyobtained.

In the arcmelting operation, the theoretical limitation due to hydrogensolubility equilibrium dictates that the pressure must be below 15 mm.Hg abs. if the hydrogen content of the titanium is to be less than p. p.m. Actually 15 mm. Hg abs. or more of argon are sufiicient to causeconsiderable heat loss by thermal conduction and convection in additionto radiation. Therefore, the upper limit of operability appears to beabout 1 mm. Hg abs. It is preferred to operate at a pressure: of lessthan 10 microns, at which pressure a hydrogen content of less than 50 p.p. m. is easily attained.

Skull melting has the advantage that scrap of unspecified shape and sizecan be readily converted to a single massive ingot. In consumableelectrode melting, some scrap can be incorporated in the electrode butthe size, shape and distribution must be carefully controlled, makingthis technique of scrap recovery difiicult and expensive. On the otherhand, scrap of any reasonable size can be charged to a skull furnace andremelted simply and economically to an ingot. Virgin metal can be meltedand alloying additions made to the skull melt in the liquid state toyield a homogeneous ingot of required composition. The only possibilityfor inhomogeneity comes from ingot solidification-segregation to whichall metals and alloys are subject in varying degrees.

The use of skull melting has made possible the replacement of alloyblending, briquetting and first melting in making melts from virginmaterials. No blending or v 3 hriquetting is required since the alloysblend in the liquid state as the melting proceeds.

There is set forth below an example which is to be construed in anillustrative, and not a limiting, sense:

7 V v Example! A-mixture of 1162 grams titanium, 623 grams -zirconiumand 623 grams vanadiumwas melted in'a'titanium baseskull atapproximately 1650 C. under 250 mm. Hg abs. of 'argon-using a'tungstenelectrode. The run lasted 35 minutes,

The molten metal-was poured into-a cold mold, solidified, andsubsequently -remelte'd by using the ingot formed as a-consumableelectrode. The latter run lasted about 2.5 minutes ata pressure-of lessthan 5 microns Hg abs The ingot formed weighed just less than -5 lbs.This ingot'had excellent-homogeneity and low'hydrogen content. V

Since certain changes may be madein the above process without departingfrom the scope ;of the invention herein involved, it is 'intended thatallmatter contained in the above description shall be interpreted asillustrative and not in a limiting sense. '-What is claimedis:

'1. A process for melting-and castinga highly reactive base metal suchastitanium'and zirconiumtdobtain a substantially uniform alloy whichcomprises mixing a quantity of virgin base metal, scrap base metal andat least onealloying metal in a preformed skull comprising thebasemetaL'subsequent-ly meltingthe mixture by means of anonconsumableelectrode to form a single 'molten pool of -a homogeneous alloy,the-formation of the-molten pool being carried out under an inertatmosphere, pouring the -molten alloy into'xa. mold to a consumableelectrode of the alloy, and are melting the consumable electrode at-apressure below 1 mmJ'Hgabs. in a cold mold to produce a substantiallyhydrogen-free, homogeneous alloy ingot.

2. A process for melting and casting a highly reactive metal such astitanium 1 and; zirconium which comprises mixing a quantity of virginmetal and scrap metal in a preformed skull comprising the metal,subsequently meltn h br us i a mm nwme k sle tredq 9 ton e s n e m n p om teLtheiormationwf the molten 9001 being. carried outnnder an inertatmosphere, pouring the molten metal into a mold to form a consumableelectrode, and are melting the consumable electrode at a pressure below1mm, Hg abs. in a cold mold to produce a substantially hydrogen-free,homogeneous metal ingot. 7

References Cited in the file of this patent UNITED STATES PATENTS OTHER,REFERENCES- Miller: The Iron Age, Septemberg23, 1954, BrqwnrThe IronAge, October 1 6, 19,52.

1. A PROCESS FOR MELTING AND CASTING A HIGHLY REACTIVE BASE METAL SUCHAS TITANIUM AND ZIRCONIUM TO OBTAIN A SUBSTANTIALLY UNIFORM ALLOY WHICHCOMPRISES MIXING A QUANTITY OF VIRGIN BASE METAL, SCRAP BASE METAL ANDAT LEAST ONE ALLOYING METAL IN A PREFORMED SKULL COMPRISING THE BASEMETAL, SUBSTANTIALLY MELTING THE MIXTURE BY MEANS OF A NONCONSUMABLEELECTRODE TO FORM A SIMPLE MOLTEN POOL OF A HOMOGENEOUS ALLOY, THEFORMATION OF THE MOLTEN POOL BEING CARRIED OUT UNDER AN INERTATMOSPHERE, POURING THE MOLTEN ALLOY INTO A MOLD TO FORM A COMSUMABLEELECTRODE OF THE ALLOY, AND ARC MELTING THE COMSUMABLE ELECTRODE AT APRESSURE BELOW 1 MM. HG ABS. IN A COLD MOLD TO PRODUCE A SUBSTANTIALLYHYDROGEN-FREE, HOMOGENEOUS ALLOY INGOT.